The present invention relates to a method for adjusting the point of engagement of a friction clutch of a step-variable transmission for a motor vehicle, in particular a friction clutch of a dual clutch transmission.
In drivetrains comprising a spur-gear transmission a friction clutch, which has the function of a drive-away and disconnect clutch, is generally arranged between the spur-gear transmission and a prime mover (such as an internal combustion engine). In the case of dual clutch transmissions a dual clutch arrangement is provided between the two transmission units and the prime mover. These friction clutches may be embodied as dry or wet-plate clutches and are subject to a certain degree of wear. In conventional manual-shift transmissions the increasing clutch wear is compensated for by a modified actuation of the clutch on the part of the driver.
In automated spur-gear transmissions (such as automated shift transmissions (AST) or dual clutch transmissions (DCT)) the friction clutch is generally actuated by an associated clutch actuator. The clutch actuator may be a hydraulic or an electromechanical actuator, for example.
The actuator may furthermore be pressure-, power- or path-controlled.
As a powershift clutch, the friction clutch is also designed to transmit, at least briefly, high torques which are equal to or generally even greater than the torque delivered by the prime mover. For a comfortable engagement, the friction clutch has to be brought from the opened state into the closed state by way of a slipping state. In so doing the input element and the output element of the friction clutch bear against one another, a torque being transmitted from a certain point in time onwards. As the point of engagement of the friction clutch, at which a certain transmission of a torque is possible, a point is defined (a control variable of the clutch actuator), at which the friction clutch transmits a specific, relatively small torque, for example a value <20 Nm.
For reliable, comfortable and also rapid gear shifts it is very important, in the case of automated step-variable transmissions, to know at what control variable of the clutch actuator this point of engagement is reached. As mentioned above, in the course of time this may vary due to wear (of the clutch plates, for example), mechanical tolerances or other influencing variables (for example settling of the clutch springs). Even relatively transient variable parameters, such as the temperature, for example, play a part in this.
In the prior art various methods have been disclosed for adjusting the point of engagement of a friction clutch.
DE 196 52 244 A1 relates to a method for kiss point adaptation, in which the engine torque is measured as an important parameter. One basic approach is to run to multiple operating points in order to obtain a reliable adaptation by averaging.
WO 2004/076224 A1 discloses a method for determining the torque transmitted by a friction clutch by comparing the rotational speeds of the engine and the transmission input, taking into account the synchronization force, particularly in overrun conditions as the engagement force rises.
WO 2004/076225 A1 discloses a characteristic curve adaptation with the engine running, the clutch being closed at a defined value, before progressively synchronizing a gear and then determining the synchronization control variable at which the rotational speeds of the transmission input and the engine diverge.
EP 0 931 961 A1 relates to a method of calibration for a control clutch at constant engine speed, the clutch calibration value being obtained from a target speed adjustment, which is determined from a profile resistance time. The profile resistance time represents a speed adjustment varying as a function of the friction characteristic, the clutch in particular being opened and closed and a measurement being taken of the time within which the speed of a gearwheel varies by a specific amount.
A similar, iterative method is disclosed by EP 0 859 171 A1.
DE 195 40 921 A1 is concerned with the idea of arbitrarily activating a clutch in order to establish the correlation between the clutch torque and a control variable.
A method for controlling the torque transmission is furthermore disclosed by DE 199 39 818 C1. The speed of travel due to the engagement of one clutch should here not be equal to zero, a parallel clutch being kept disengaged. The point of engagement of the parallel clutch is determined through at least partial engagement of this clutch, a measurement being taken of the variation in the speed of one clutch half per unit time.
DE 102 44 393 A1 relates to the determination of the point of engagement through measurement of the pressure profile and determination of the first derivative of this on a hydraulic cylinder.
DE 100 54 867 discloses a method for determining a creep point of a friction clutch. A first creep point is set and the clutch torque is measured. Failing agreement in the first step, a second creep point is then set. A value situated between the slip point and the first creep point is here selected as reference point. The new second creep point is then set so that the desired torque and the torque actually transmitted correspond.
DE 101 13 700 A1 discloses a method for determining an engine friction torque. The method is used to determine the torque applied to the friction clutch from the engine torque.
A method for determining a biting point of a friction clutch is furthermore disclosed by EP 1 741 950 A1. A friction clutch is opened so that the transmission input speed falls. The clutch is then closed again until the fall in the speed is arrested. The clutch is then closed further until the speed remains constant or falls. The biting point is calculated from this.
DE 101 01 597 A1 discloses a method for gearshift control, in which the engine torque and the clutch torque are controlled. The control varies as a function of speed differentials.
DE 197 51 455 A1 relates to a method for clutch control, a set clutch capacity being adjusted by a control unit on the basis of a characteristic function. The characteristic function is here continuously adapted.
DE 102 24 064 A1 discloses a method for changing gear. In this case a synchronization is cancelled through the determination of a ‘cut-off point’, which varies as a function of a speed gradient of the synchronized gearwheel.
Finally, EP 1 067 008 A1 discloses a method for clutch characteristic curve adaptation, for a dual clutch transmission.
The intention here is to adapt the clutch characteristic curve of whichever clutch is currently not being used to transmit a torque (the so-called free friction clutch). This is done by first closing the clutch with a specific control force and waiting until the transmission input shaft has reached the synchronous speed (that is to say it is turning together with the engine shaft). A synchromesh of the associated free transmission unit is then actuated until a sufficient speed differential prevails. This synchromesh is then released and the speed gradient of the transmission input shaft is then determined. The value of the previously transmitted clutch torque is then calculated from this. On the basis of this value a clutch characteristic curve adaptation is then performed in connection with the previously established clutch control force.